Accurately monitoring of the location and flow of the objects associated with inventory, product manufacturing, merchandising, and related operations is challenging. There is a continuing need to determine the location of these objects and to track relevant information about the objects. A tag, marker or label device suitably configured to be associated with any of a variety of objects, including goods, items, persons, or animals, or substantially any moving or stationary and animate or inanimate object, which facilitates location and data tracking, can be used. One such tag tracking system is an electronic identification system, such as RFID. RFID tags are affixed to, connected to, or in some way associated with an object for the purpose of tracking the object, and storing and retrieving information about the object.
The RFID tag stores data associated with the object. An RFID reader may scan for RFID tags by transmitting an interrogation signal at a known frequency. The RFID tags may respond to the interrogation signal with a response containing, for example, data associated with the object or an RFID tag ID. The RFID reader detects the response signal and decodes the data or the RFID tag ID. The RFID reader may be a handheld reader, or a fixed reader by which items carrying an RFID tag pass. A fixed reader may be configured as an antenna located in a pedestal similar to an electronic article surveillance (“EAS”) system.
Antennas collect and emit energy in the form of electromagnetic waves. The units for this transfer take the form of power-per-unit area. Many tags for use in such tag detection systems have a single favored orientation with respect to the stimulating field where they exhibit a maximum response, i.e., they are directional. Most tags are somewhat rectangular in shape and are variations of a dipole antenna, with a high length-to-width ratio. These tags give a maximum response when oriented within an incident field orthogonal to the long axis of the tag. This property is commonly referred to as “read orientation sensitivity”.
For example, FIG. 1 illustrates an example of a RFID tag 100 with an antenna 102 disposed upon substrate 104. Substrate 104 is substantially rectangular in shape. The antenna 102 comprises multiple antenna portions, i.e., antenna 102 has a first antenna portion 106 and a second antenna portion 108. The first antenna portion 106 is connected to a first side 112A of lead frame 112. Second antenna portion 108 may be connected to a second side 112B of lead frame 112. RFID chip 110 may be connected to lead frame 112 by ultrasonically bonding lead frame 112 to the conductive pads on RFID chip 110. RFID chip 110 and lead frame 112 are placed directly in the geometric center of the dielectric substrate material of substrate 104. The ends of lead frame 112 can be physically and electrically bonded to the foil antenna pattern of antenna 102. The RFID chip also can be bonded directly to antenna 102 at the conductive pads by use of conductive adhesive to eliminate the need for lead frame 112.
The first antenna portion 106 has a first antenna end 106A and a second antenna end 106B. Similarly, second antenna portion 108 has a first antenna end 108A and a second antenna end 108B. The first antenna end 106A of first antenna portion 106 is connected to lead frame 112A. First antenna portion 106 is disposed on substrate 104 and forms an inwardly spiral pattern from RFID chip 110 in a first direction, with second antenna end 106B to terminate on the inner loop of the inwardly spiral pattern on one half of the substrate 104. Similarly, first antenna end 108A of second antenna portion 108 is connected to lead frame 112B. Second antenna portion 108 is disposed on substrate 104 to form an inwardly spiral pattern from RFID chip 110 in a second direction, with second antenna end 108B to terminate on the inner loop of the inwardly spiral pattern on the other half of the substrate 104. As illustrated in FIG. 1, the two clockwise spiral sections 106, 108 of antenna 102 basically are rotationally symmetrical with respect to each other. The RFID tag 100 generates a radiation pattern 200 (FIG. 2) similar to the radiation pattern of a conventional dipole antenna.
The RFID tag 100 receives and emits best when perpendicular (e.g., along the z-axis) to its y-axis and not at all along that y-axis (also referred to as the “dipole axis”), as illustrated by the radiation pattern 200 graph of FIG. 2. The dead area in the radiation pattern 200 of the antenna 102 is referred to as a null 202. Antenna directivity is important for RFID tags because if the tag 100 is oriented where its null 202 is pointed at the tag reader, the tag 100 receives no power for excitation and therefore is not read. In general, the radiation pattern describes the sensitivity of the receiving antenna to the direction of travel or the propagation of an electromagnetic (“EM”) wave. Since the EM wave is a transverse wave, the E-field component of the EM wave is perpendicular to the direction of the wave propagation.
Another situation that causes additional null regions in the radiation pattern 200 of the tag antenna 102 is when the RFID tag 100 is applied to a conductive surface, e.g., a metal surface. In order to couple energy into a “dipole-like” antenna, an excitation field (“E-field”) parallel to the length of the dipole-like antenna that has the proper frequency is required. The conductive nature of the metal dictates that the tangential e-field, which is aligned with length of the dipole, will be zero on the metal surface. This effect prevents coupling of energy into the RFID tag 100, which causes a full or partial degradation of the detection performance of the RFID tag 100.
Once removed from the surface of the metal, the electric field can be non-zero. Therefore, a dielectric spacer, which provides separation between the dipole antenna and the metal surface, enables some degree of an excitation field reaching the RFID tag 100. However, a large spacer, e.g., larger than ten millimeters, is required even for ultra-high frequency (“UHF”) RFID tags to regain comparable exposure to the excitation field, and thus making the packaging and application impractical. In addition, the typical dielectric spacer is relatively expensive.
In view of the above, it is desirable to provide an RFID device having a radiation pattern that is minimally affected by a conductive surface, such as a metal surface, EAS tag, etc.